This invention relates generally to warp knitting machines, and more particularly to a method and an apparatus for supplying a multi-layer, multi-axial yarn stratum to the knitting site of a warp knitting machine which is equipped with endless longitudinal conveyors running toward the knitting site with their upper strands on either side of the knitting site and having chains which are edged with hooks for guiding the yarn stratum so that at least two multiple yarn guides for each yarn sheet are located over both conveyor chains and are changeably movable, with at least one of the multiple yarn guides having a central movement path inclined at an acute angle to the direction of motion of the conveyor chains wherein the stratum is fed between the conveyor chains by means for supporting the yarn.
An apparatus of the type described above is taught in U.S. Pat. No. 4,325,999. Several layers of parallel yarn sheets are deposited between endless conveyor chains, which are disposed parallel to one another and the upper strands of which are moved in the cast-off plane of the knitting site toward the latter. The sum of all yarn layers or yarn sheets is referred to as a yarn stratum.
The endless conveyor chains are equipped with hooks, in which several multiple yarn guides or filling yarn guides, which can be moved in different directions to the conveyor chains, suspend yarn sheets consecutively and one over the other.
Depending on the number and the respective density of these yarn sheets and depending on the thickness of the yarns or fibers contained therein, a yarn with a considerable mass may result.
With the objective of preventing sagging of this heavy yarn stratum, the conveyor chains are provided with a stable lateral guide, particularly in the region of their upper strand.
However, with such a lateral guide, the yarn stratum cannot be positioned precisely horizontally before it reaches the knitting site of the warp knitting machine. The conveyor chains can absorb only limited forces laterally. Under the possible high load of yarns, the hooks can deform or break.
In order to lessen the effect of this problem, the manufacture of such knitted fabrics has, in the past, been limited to a certain width. The sag of the yarn sheets, which is nevertheless unavoidable, has been countered by providing the inlet region of the knock-over bits of the knitting site with large, arc-shaped inlet sections. Such arc-shaped inlet sections compensate for the sag in the yarn stratum and enable it to be brought straight over the whole of the working width to the cast-off plane, which is indispensable for the knitting process.
Such arch-shaped inlet sections can compensate for only slight sags, and, especially in the middle region, lead to a clear shifting of the lower layer of the yarn stratum. This shifting is fixed by the knitting process that follows immediately, and remains as a defect or irregularity in the goods.
Even if such irregularities can be tolerated in the finished knitted fabric, such fabrics can be produced in only relatively narrow widths.
If special runs of knitted fabric are to be produced by this method and not limited in width to the previously customary 50 inches, then such a method is not at all practicable. When production of wider fabrics is attempted, a reliably consistent sag of the yarn stratum cannot be ensured merely by the lateral guidance of the conveyor chains with a corresponding absorption of forces. The hooks at the conveyor chains would also no longer withstand the forces necessary for stretching the yarns.
Other disadvantages become apparent if, in accordance with U.S. Pat. No. 4,325,999, supporting elements in the form of a stretched sheet, guided over rollers, are disposed below the yarn strata. At the knitting site, this sheet is combined with the yarn stratum and remains a component of the fabric. This is highly unsatisfactory, if the sheet is not required as a component of the yarn stratum.
Without reference to the problem above, it is proposed in DE 33 048 A1 that so-called stationary yarns be supplied between each layer of yarns. Such stationary yarns, which can be stretched between their feed roller and the knitting site, can support the layer of yarns placed over them and minimize the sagging of this yarn stratum. In this case also, these additional, supporting yarns are tied into the knitted fabric. They lead to a larger, generally unnecessary thickness of the knitted fabric. In most cases, the structure build-up required excludes the use of this method. Higher costs and a higher weight are an inevitable consequence.